While these various disturbances cannot be controlled, it is possible to detect in real time that the GNSS data is outside of limit values, and compute in real time a corresponding integrity level, by the use of algorithms ensuring a confidence interval on the position information generated by the system. In particular, various algorithms corresponding to the different GNSS configurations have been defined, even standardized, such as the “HPL/VPL” algorithm in the case of a GNSS configuration using GPS and GBAS.
Also, the GNSS-based navigation systems provide for the detection of a data integrity fault, to switch off the system if necessary: an integrity problem is thus transformed into a continuity problem, since the GNSS data is no longer available.
In parallel, solutions of hybridation with the inertial reference system IRS of the aircraft have been proposed, to overcome this continuity problem, so as to supply guidance signals in all circumstances, having the requisite accuracy and integrity.
An inertial reference system is commonplace in aircraft. It comprises gyrometers and accelerometers, and digital processing means for integrating the data from these sensors. Preferably, it also includes anemometers, which are used to obtain the vertical stability of the inertial system. The inertial reference system supplies in particular position, velocity, angular velocity, acceleration and attitude data in various fixes associated with the carrier or the ground. It can also supply information such as the wind speed, magnetic heading, etc.
The principle of a hybrid GNSS-IRS system is as follows: the navigation system uses the extremely accurate (augmented) GNSS data, provided it has sufficient integrity. The data is then also used to compute long term drifts (sensor bias) of the IRS unit, so that this unit can be realigned. When the integrity of the GNSS data is no longer sufficient, the GNSS channel is cut. The navigation system then uses the inertial position/velocity information from the IRS platform.
Since the IRS system is intrinsically less accurate than a GNSS type system, because of the accumulation over time of errors on the information output by the inertial sensors (scale factor errors, bias errors, variations with temperature, etc), its realignment by very accurate PVT information supplied by the GNSS system provides for inertial information with sufficient accuracy while the GNSS system is switched off, since sensor drift is only a factor in the long term.
Thus, a GNSS (GBAS augmented)-IRS hybrid system can be used to provide, in all circumstances, in all weathers, PVT data with the requisite level of integrity and continuity, so that its use for precision (category III) approach operations can be considered. The requisite integrity and continuity levels are achieved through the real-time realignment of the inertial reference system, and the switchover from the GNSS system to the realigned IRS system in the event of GNSS data integrity problems. In particular, numerous data hybridation methods with filtering and other hybridation methods have been developed to enhance these GNSS-IRS hybrid systems, in particular to detect the GNSS data integrity problem early enough for it not to cause an erroneous correction of the bias errors in the inertial reference system.
This data supplied by the GNSS-IRS hybrid system is used, together with the GBAS augmentation signals supplied by a ground station provided in the airport concerned (or the SBAS augmentation signals), to generate guidance signals for approach operations relating to this airport.
Such a system is called a GNSS-based landing system (GLS), augmented by inertial equipment.
Such a GLS system indeed offers many advantages over the ILS and MLS systems currently used for precision approach operations. In particular, the satellite signals are available everywhere, the GBAS (or SBAS) differential augmentation is used to obtain the necessary accuracy and the operational maintenance of such a system is made much easier, because it is almost entirely digital.
The ILS and MLS landing aid systems do pose a practical problem of cost, both for purchase and maintenance. In particular, protecting an ILS or MLS ground station against the interference factors hampering data transmission increases the running costs of these systems. In particular, to ensure the high level of integrity and continuity of the guidance data required in low visibility conditions, the use of the ILS or MLS systems is subject to draconian operating restrictions. Also, in low visibility situations, the number of guidance operations per hour performed in an airport drops accordingly. This poses a major problem in all airports where poor weather conditions are commonplace.
For all these reasons, a GLS system is theoretically very attractive compared to the ILS or MLS systems.
One object of the invention is to propose a GLS system based on a GBAS (or SBAS) augmented GNSS system and a GNSS-IRS hybridation.
Another object of the invention is to use an existing navigation and guidance architecture, with market-standard equipment, in particular multimode receivers MMR of the state of the art, in order to propose an inexpensive GLS system.